The present invention concerns a method for production of magnesium chloride of sufficiently high purity for production of magnesium metal, by dissolving magnesium containing ore in hydrochloric acid and with subsequent purification of the raw solution by precipitation of unwanted impurities.
Magnesite from different sources and of different qualities might play an important part in the future production of magnesium metal-based on this method. Its content of impurities shows great variations, concerning the most critical elements for a subsequent electrolytic metal production. Iron can be found in concentrations between 0.012 and 2-3%, nickel in the range from 3 to 180 ppm and phosphorus from 5 to 500 ppm in the ores.
Another possible source for the magnesium chloride production is brucite (Mg(OH).sub.2). Its purity is comparable to good qualities of magnesite. Olivin, a magnesium silicate, has a potential as a raw material as well as precipitated Mg(OH)2 from the seawater/dolomite process. Also, asbestos tailings could be used for this purpose.
The requirements for a high current efficiency as well as existing specifications for the metal, demands a high degree of purity in the purified brine product. For example the nickel content must not exceed 0.1-0.2 ppm if the corrosion properties shall be maintained. The phosphorus content must be kept below 1 ppm to avoid the formation of malodorous phosphines from moistened metal surfaces.
The invention applies to the first stage in a dissolution process, e.g. as it is described in Norwegian patent No. 161 851. According to this patent a first reactor is filled with magnesite lumps (5-400 mm). The magnesite is dissolved by feeding hot, concentrated hydrochloric acid into the bottom of the reactor, and the resulting solution is drained off some distance below the top, in such a way that a layer of magnesite is situated above the fluid level. Magnesite lumps (5-400 mm) are fed to the top of the reactor as it is consumed and sink downwards in the reactor.
The dissolution reaction is finished in the second reactor. The composition of the solution is adjusted by supplying either finely crushed magnesite if the solution contains an excess of unreacted acid, or concentrated hydrochloric acid if the solution has an excess of magnesite particles.
Further in the process, iron and other heavy metals are removed after oxidation, by precipitation as hydroxides.
Magnesite is one of the ores which is preferred as raw material for production of magnesium chloride according to this process, but other magnesium containing ores could also be used. As earlier said, magnesite is present in nature in different qualities and of different reactivity, dependent on the place of origin. The magnesite can be divided into two main types of ores:
Macrocrystalline magnesite consists of aggregates of single crystals with a diameter greater than 1 mm, and cryptocrystalline has grains smaller than 1/100 mm. While the first type is found as lodes in sedimentary rocks, the cryptocrystalline magnesites are present as single nodules in sand- or clay-containing materials.
As the grain boundaries seem to be the first to be attacked during the dissolution process, resulting in liberation of the individual crystals, there are great differences up to 1000 times) in the reactivity between macro- and cryptocrystalline material.
The two types react differently during the dissolution process and both have advantages and drawbacks as raw material for the magnesium chloride production.
Macrocrystalline magnesites react relatively slow with hydrochloric acid. To obtain acceptable production loads and degree of conversion, the acid must in practice be preheated so that the reaction can be carried out at temperatures between 70.degree. and 100.degree. C. The capacity of the reactor is thereby limited to what quantity of hydrochloric acid vapor can be accepted, which must be recovered from the carbon dioxide.
Several of these magnesites have a low content (1-2%) of components being insoluble in acid. These are partly present as separate single crystals in the size of about 1 mm, and which therefore to a great extent are transported out with the fluid flow. Thereby the reactor should have a reasonably long operating time before it must be emptied of insoluble particles.
Cryptocrystalline magnesites react as earlier mentioned, much faster (1000 times) than the macrocrystalline. The reaction can therefore be carried out at a lower temperature. The solution leaving the reactor is therefore practically neutral, so that the acid losses in the waste gas are negligible. The single crystals, however, are liberated substantially faster than they are consumed by the reaction. The solution leaving the reactor can contain a quantity of the liberated mini particles, which dependent on working conditions and flow, can correspond to until 50-100% of the quantity which has reacted with the acid.
To utilize the magnesity grains in this suspension a corresponding quantity of acid must be added to the second leaching reactor. Such a high content of mini crystals easily leads to exceeding the reactor capacity, leading to overfoaming caused by liberated carbon dioxide, a phenomena which is increased because the particles stabilize the foam production.
Another disadvantage of cryptocrystalline magnesites, because of their origin, is that they contain variable quantities of silicates which react with the acid under formation of hydrated silica, partly in the shape of a voluminious sludge which follows the solution and is removed together with the heavy metals in the filter after the purification process, and partly form porous particles of SiO.sub.2 having a size of until up to several cm in diameter.
While the SiO.sub.2 sludge at worst gives a certain lowering of the filterability, the greater SiO.sub.2 -frameworks will lead to the reactor becoming rapidly filled with undissolved components, resulting in short operation periods between each time the reactor must be emptied and cleaned. This also could be the result when using other magnesium containing raw materials with a high content of silica.
A further defect with these magnesite minerals is that they normally have a lower Fe/Ni-ratio than the optimal ratio for removal of nickel during the purification process, so that extra supply of iron ions will be necessary.